The invention relates to a process for the continuous heat treatment of steel strip material whose thickness (d1, d2) periodically varies along its length. Such strip material is produced continuously under the name of flexibly rolled strip with a roll gap varying as a function of time.
Flexibly rolled strip with periodically variable plate thicknesses can be produced by cold rolling. After having been cold-rolled, the strip material is normally heat-treated in order to reduce the amount of strain hardening introduced during cold rolling and thus to improve again the plastic deformability of the strip material. It is known to heat complete coils in high-convection hood-type annealing furnaces. However, the annealing cycle in such cases is very long because the material is treated at heating rates of less than 0.1 K/sec and cooling rates of less than 0.01 K/sec.
However, for producing the polyphase structure, polyphase steels require rapid heating and cooling rates which cannot be achieved by high-convention hood-type annealing furnaces. Therefore, polyphase steels when passing through the furnace have to be heat-treated continuously. In the process, the strip material is unwound from the coil and the strip, in one or several furnaces adjusted to one another, i.e. mainly radiation and high-convection furnaces. The strip material is heated as far as the αγ-range and held prior to being quenched in a high-speed cooling device, whereby, provided a suitable strip material quality has been selected, the different phase portions of a polyphase steel can be formed.
Induction heating forms part of the electrical heating processes wherein the thermal energy is produced by resistance heat directly in the workpiece or strip in this case. In the case of longitudinal field heating, the magnetic field extends parallel to the surface of the strip, as a result of which eddy currents are induced in the strip. The eddy currents also extend parallel to the strip surface on both sides of the strip in the opposite directions. Eddy currents can lead to the formation of internal heat sources which heat the strip. The magnetic alternating field and the current density decrease from the material edge towards the strip interior. The penetration depth δ describes the distance from the material edge at which the magnetic field has been reduced to 63% of the outer field.
The penetration depth δ thus describes the distance from the surface at which amplitudes of the magnetic and electric field strength have decreased by 37%, i.e. by the factor 1/e. In the case of a thin strip, the operation takes place mainly with penetration depths δ<0.4 d (d=thickness of strip), so that the currents inside the strip do not cancel each other out. The heat is thus generated in an outer edge layer and, through heat conduction, reaches the strip interior. The penetration depth δ can be set via the selection of the frequency f.
The penetration depth σ is calculated as follows:
  δ  =            1              2        ⁢        π              ·                            ρ          ·                      10            7                                    f          ·          μ                    with ρ=specific electric resistance                f=frequency        μ=permeability.        
When strip material with a periodically variable thickness, i.e. flexibly rolled strip, is subjected to this kind of heat treatment, a problem can occur in that the strip regions with different thicknesses can become heated at different speeds in radiation and convection furnaces. With a basically identical heat input rate inside the furnace, the temperature of the thin strip portions is substantially higher at the furnace exit than that of the thicker strip portions. In principle, this also applies to induction furnaces in which, by selecting a certain frequency, a certain penetration depth of the induced current can be generated. This penetration depth can be identical in thick and thin strip portions, with the heat input being approximately proportional to the penetration depth δ. Accordingly, the thinner strip portions, due to their smaller mass, can be heated to a higher temperature than the thicker strip portions.